Engine Coolant Temperature (ECT) Sensor Circuit High
In short
P0118 means the engine coolant temperature sensor signal is too high, which the computer reads as an impossibly cold (or 'open circuit') temperature. The usual causes are a failed ECT sensor, a broken or corroded connector/wiring (an open circuit reads high), or rarely a bad ground. Because the computer thinks the engine is freezing, it over-fuels — hurting fuel economy and sometimes causing hard starting and a non-functional gauge.
Is it safe to drive with P0118?
Generally drivable, but not ideal. With the computer thinking the engine is ice-cold, it runs a rich warm-up mixture continuously — wasting fuel, fouling plugs over time, and possibly causing a no-cold-start-issue or black smoke. Your temperature gauge may also be wrong, so you'd miss a real overheat. Fix it soon; it's usually cheap.
Symptoms
- Check engine light on
- Temperature gauge reads low/cold or doesn't move
- Poor fuel economy and possible black smoke (rich running)
- Hard starting, especially when warm
- Cooling fans may run constantly or not as expected
Common causes (most → least likely)
How to diagnose it (before buying parts)
- 1 Locate the ECT sensor (note: this is the sensor that feeds the computer, which may be separate from the gauge sender). Inspect its connector and wiring for damage, corrosion, or coolant intrusion.
- 2 With a scan tool, read the ECT value: a reading pinned at a very low temperature (e.g. -40°F) on a warm engine indicates a high/open signal.
- 3 Unplug the connector and check resistance across the sensor against a temperature-resistance chart; an infinite/open reading means a failed sensor.
- 4 Back-probe the connector: with the sensor connected and engine warm, the signal voltage should be low. A near-reference-voltage signal points to an open in the sensor or wiring.
- 5 Jumper the signal and ground at the connector (per the service procedure) — if the reading swaps to hot, the wiring is good and the sensor is bad.
- 6 Repair any broken wire, bad ground, or corroded connector; otherwise replace the ECT sensor.
Repair options & cost
By manufacturer
ECT sensors are inexpensive and a common failure; check the connector for corrosion. Replacing the sensor usually resolves P0118.
Connector corrosion and sensor failure are typical; confirm with resistance vs temperature before replacing, and inspect for coolant in the connector.
Usually a failed sensor after years of heat cycling; use an OEM sensor and check the harness connector for brittleness.
The combined coolant temp sensor (often green-topped) is a known failure item; replace with the updated OEM part and a new connector/clip if brittle.
Frequently asked questions
What does 'circuit high' mean for a temp sensor?
Coolant temp sensors lower their resistance as they heat up. A 'circuit high' (P0118) means the signal voltage is high — the resistance looks extremely high or open — which the computer interprets as an impossibly cold engine. An open wire or a failed sensor both produce this.
Why does my car run rich with P0118?
The computer thinks the engine is freezing cold, so it commands a rich warm-up mixture all the time. That wastes fuel, can foul spark plugs, and may cause black smoke or hard warm-starts until the sensor reads correctly again.
Is it the sensor or the wiring?
Either. An open or corroded connector/wire produces the same 'too cold' high signal as a failed sensor. Check the connector and wiring first (cheap), then test the sensor's resistance against a temperature chart before replacing it.
Can I drive with P0118?
Short-term yes, but your temperature gauge may be inaccurate, so you could miss an actual overheat, and the engine runs rich. It's an easy, inexpensive fix — do it soon.